Ultrasonic, flow measuring devices are applied often in process and automation technology. They permit easy determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic, flow measuring devices frequently work according to the Doppler principle or according to the travel-time difference principle. In the travel-time difference principle, the different travel times of ultrasonic pulses are evaluated as a function of flow direction of the liquid. For this, ultrasonic pulses are sent at a certain angle to the tube axis both in, as well as also counter to, the flow direction. From the travel-time difference, the flow velocity, and therewith, in the case of the known diameter of the pipeline cross section, the volume flow, can be determined.
The ultrasonic waves are produced, respectively received, using ultrasonic transducers. For this, ultrasonic transducers are secured in the tube wall of the relevant pipeline section. Another ultrasonic, flow measuring device, which works according to the travel-time difference principle, is known from U.S. Pat. No. 5,052,230. The travel time is, in such case, ascertained by means of short ultrasonic pulses, so-called bursts.
The ultrasonic transducers are normally composed of an electromechanical transducer element, e.g. a piezoelectric element, an ultrasound window, also called a coupling layer, and a housing. The piezoelectric element is composed in industrial process measurements technology usually of a piezoceramic. The ultrasonic waves are produced in the piezoelectric element and led via the ultrasound window into the measured medium. Arranged between the piezoelectric element and the coupling element can be another coupling layer, a so called adapting, or matching, layer. The adapting, or matching, layer performs, in such case, the function of transmitting the ultrasonic signal and simultaneously the reduction of a reflection on interfaces between two materials caused by different acoustic impedances.
More and more, ultrasonic transducers are also being applied for flow measurement of gases. In gases, the transmission of sound energy usable for measuring is essentially smaller than in liquids.